1. Field of the Invention
The present invention relates to a bobbin unit for holding the field coil of a brushless alternator.
2. Description of the Related Art
FIG. 13 is a sectional view showing a conventional brushless alternator for a vehicle taken along the axial direction thereof, FIG. 14 is an upper plan view of the iron core of a conventional bobbin unit, FIG. 15 is a sectional view taken along the line XVxe2x80x94XV of FIG. 14, FIG. 16 is an upper plan view of the plate of the conventional bobbin unit, FIG. 17 is a sectional view taken along the line XVIIxe2x80x94XVII of FIG. 16, FIG. 18 is an upper plan view of the bobbin of the conventional bobbin unit, FIG. 19 is a sectional view taken along the line XIXxe2x80x94XIX of FIG. 18, FIG. 20 is a backside plan view of the bobbin and FIG. 21 is a sectional view of the main portion of the bobbin unit showing how an iron core is connected to a plate by spot welding.
In FIG. 13, a brushless alternator 1 is arranged such that a field coil 2 does not rotate and a bobbin unit 6 which holds the field coil 2 is fixed to the rear bracket 7 of the alternator, and is different from an alternator with a rotary field type brush. An N-pole rotor core 8 and an S-pole rotor core 9 are rotated together with a shaft 10.
The bobbin unit 6 is composed of an approximately cylindrical iron core 3 as a field iron core forming a magnetic circuit, a thin wall plate 4 connected to the iron core 3 by spot welding and a resinous bobbin 5 interposed between the iron core 3 and the plate 4 for accommodating the field coil 2.
As shown in FIG. 14 and FIG. 15, the iron core 3 is formed of a cylindrical thick wall portion 3a having a thick wall and a thin wall portion 3b projecting from the minimum radius portion of the thick wall portion 3a along the periphery thereof. The iron core 3 is made by being machined from a round steel bar by turning. On the other hand, the plate 4 is stamped from a mild steel sheet and composed of a cylindrical portion 4a formed to a cylindrical shape and a flange portion 4b formed from one end of the cylindrical portion 4a so as to spread outwardly of the radial direction thereof is shown in FIG. 16 and FIG. 17. Cutouts 4c, which position a bobbin 5 to be described later and with which the projections of detents are engaged, are formed to the outer periphery of the flange portion 4b. 
As shown in FIG. 18 to FIG. 20, the bobbin 5 is formed of an approximately-thread-spool-shaped cylindrical portion 5a formed to a cylindrical shape and flange portions 5b, 5c formed at both the ends of the cylindrical portion 5a. Projections 5d which are engaged, as detents, with the cutouts 4c of the plate 4 are formed to the outer periphery of the flange portion 5c on one side. On the other hand, a projection 5e having a through hole at the center thereof is formed to the outer periphery of the flange portion 5b on the opposite side and serves as a detent also acting as a port through which an electric cable is drawn out. Further, four through holes 5f are defined along the circumference of the cylindrical portion 5a at the intervals of 90xc2x0. The through holes 5f are defined to permit electric poles to pass therethrough when the iron core 3 is connected to the plate 4 by spot welding.
The conventional bobbin unit 6 arranged as described above is assembled in such a manner that, first, the cylindrical portion 4a of the plate 4 is inserted into the bobbin 5 and positioned thereto by the engagement of the projections 5d with the cutouts 4c. Next, the end of the cylindrical portion 4a opposite to the flange portion 4b is mounted to the thin wall portion 3b of the iron core 3 and the thin wall portion 3b and the cylindrical portion 4a are clamped by a pair of electrodes disposed on the bobbin 5 side and the inner peripheral side of the thin wall portion 3b and spot welded, so that the iron core 3 is connected to the plate 4. Thereafter, a not shown tape is wound around the cylindrical portion 5a of the bobbin 5 for clogging the through holes 5f in order to prevent the short circuit of the field coil 2.
In the conventional bobbin unit 6 arranged as described above, the field coil 2 is wound around the bobbin 5 of the bobbin unit 6 and fixed to the rear bracket 7 of the brushless alternator 1.
In general, since the smaller winding diameter of the field coil 2 can shorten the length of a copper wire even if the wire has the same number of turns, a cost can be reduced. Therefore, it is effective that the cylindrical portion 5a of the bobbin 5 has a diameter as small as possible. In the conventional bobbin unit 6, however, since the thin wall portion 3b is required to the iron core 3 to execute the spot welding, the diameter of the cylindrical portion 5a of the bobbin 5 is increased by the thickness of the thin wall portion 3b. In addition, the thin wall portion 3b needs a predetermined length in an axial direction and must be formed to an accurate cylindrical surface. Further, since the thin wall portion 3b has a thin wall thickness, the iron core 3 is machined by turning from a round steel bar and the thin wall portion 3b is formed by removing an unnecessary portion from the round steel bar. As a result, a large amount of the steel bar is removed by the turning, by which the material and time are wasted. Further, workability is not good because the field coil 2 is wound after the iron core 3 is connected to the plate 4.
In addition, the through holes 5f must be defined to the bobbin 5 for spot welding and further the tape must be wound to clog the through holes 5f. 
An object of the present invention made to solve the above problem is to provide a bobbin unit for a brushless alternator field coil which can reduce the diameter of a bobbin, need not wind a tape and can be made easily and a method of assembling the bobbin unit.
A bobbin unit for a brushless alternator field coil according to one embodiment comprises a ring-shaped iron core having a thick wall; a plate which has a cylindrical portion and a flange portion, the cylindrical portion is formed to a thin cylindrical shape having a hole whose inside diameter is as large as the center hole of the iron core and the end of one side of the cylindrical portion is abutted against and connected by butt connection means to the end surface of the iron core on one side thereof with the axial center of the cylindrical portion being in coincidence with that of the iron core, and the flange portion extends outwardly in a radial direction from the end of the cylindrical portion on the other side thereof; and a resinous bobbin disposed in the space formed by the end surface of the iron core on the one side thereof and the outer peripheral surface of the plate for insulating and accommodating a field coil.
A bobbin unit for a brushless alternator field coil according to another embodiment is arranged such that the iron core includes a concentric circular engaging portion formed to the end surface on the side thereof, and the cylindrical portion is connected and the cylindrical portion is connected to the iron core by butt connection means with the end on the one side thereof being engaged with the engaging portion.
A bobbin unit for a brushless alternator field coil according to yet another embodiment is arranged such that the butt connection means is butt welding.
A bobbin unit for a brushless alternator field coil according to still another embodiment is arranged such that the butt connection means is projection welding.
A bobbin unit for a brushless alternator field coil according to still another embodiment is arranged such that the butt connection means is beam welding.
A bobbin unit for a brushless alternator field coil according to still another embodiment is arranged such that the butt connection means is an adhesive.
A method of assembling a bobbin unit for a brushless alternator field coil according to another embodiment comprises the steps of mounting a thread-spool-shaped bobbin to a plate composed of a cylindrical portion having a thin wall and a flange portion extending outwardly in a radial direction from the end of the cylindrical portion one side thereof from the direction opposite to the flange portion, the bobbin being as long as the cylindrical portion; and abutting the end of the plate opposite to the flange portion against the end surface of a thick ring-shaped iron core and connecting the plate to the iron core by butt connection means.
A method of assembling a bobbin unit for a brushless alternator field coil according to yet another embodiment is arranged such that the butt connection means is butt welding.
A method of assembling a bobbin unit for a brushless alternator field coil according to still another embodiment is arranged such that the butt connection means is an adhesive.